With the occupation of the Gilbert and Marshall Islands by early 1944, American attention (in their 'island-hopping' campaign in the Central Pacific) turned next to the Mariana Islands, whose airfields were in bomber range of Japan. Operation Forager was launched on 15 June 1944 with the invasion of Saipan, followed on the 21 July with the invasion of Guam and on the 24 July with the invasion of Tinian. The fighting however, was more costly and protracted than predicted, the Americans suffering some 27,000 casualties. This led to delays in securing the Marianas (which were not secured until the beginning of August), that had three immediate impacts upon the plan to attack the Palau Islands (Operation Stalemate), scheduled for early September 1944 as part of the plan to support General MacArthur's drive in the Southwestern Pacific:

Firstly, it delayed the arrival of the new III Amphibious Corps commander, Major General Roy S Geiger until planning (undertaken by a temporary staff headed by Major General Julian Smith commanding Task Force 36, called X-Ray Provisional Amphibious Corps) was at quite an advanced stage and any major changes would be difficult to implement.

Secondly, it caused serious friction between the Army and the Marine Corps as Lieutenant General Holland ("Howlin' Mad") Smith had relieved Major General Ralph C Smith of his command of the 27th Infantry Division for 'defective performance'. This was to have serious repercussions all the way back to Washington DC and on the Peleliu operation, although the two formation commanders actually worked very well together.

Thirdly, it continued to tie up troops, resources and shipping (particularly the III Amphibious Corps and 77th Infantry Division on Guam and the 27th Infantry Division on Saipan). Additionally, intelligence (including the capture of the 31st Army files and a Japanese Intelligence Officer on Saipan) revealed that Babelthuap (the largest island in the Palau Island group) had only marginal utility in regard to the expansion of the airfield facilities but had a large Japanese garrison, while Peleliu already had an excellent operational airfield that once in American hands, could neutralise the northern one.

The planning was altered and the target date (for the first phase) changed to 15 September 1944 - the same day as MacArthur's forces would take Morotai. The new plan would be known as Operation Stalemate II, the first phase of which would involve the III Amphibious Corps (1st Marine and 81st Infantry Divisions) assaulting Peleliu and Angaur. The second phase would see XXIV Corps (7th and 96th Infantry Divisions) attacking the atolls of Yap and Ulithi on October 8th, while the 77th Infantry Division would become the operation's floating reserve and the 5th Marine Division act as a general reserve on Hawaii. The two phases would be supported by the US Navy's Western Pacific Task Force from the Third Fleet. The Covering Forces and Special Groups (Task Force 30) would remain directly under Halsey, the Third Amphibious Force (Task Force 31) was divided into the Western Attack Force (Task Force 32) bound for Peleliu and Angaur under Rear Admiral George H Fort and the Eastern Attack Force (Task Force 33) bound for Yap and Ulithi under Vice Admiral Theodore S Wilkinson. Task Force 32 was itself split into the Peleliu Attack Group (1st Marine Division) directly under Fort and the Angaur Attack Group under Rear Admiral H P Blandly.

Admiral William F 'Bull' Halsey, Commander of the Western Pacific Task Force, had overall responsibility for conducting supportive attacks against a number of Japanese bases both in the Palau Islands and in the Philippines. As these raids were taking place, the invasion force was heading towards Peleliu, but to Halsey's surprise, these raids were only lightly contested, making Halsey suspect that the Philippines (in particular) were not as heavily defended as first thought. He ordered his Chief of Staff, Rear Admiral R B Carney to send an urgent message to Admiral Nimitz just two days (13 September) before the assaults on Peleliu and Morotai were to take place, recommending that firstly, the assaults be abandoned, secondly, that the ground forces that were to be used be transferred to MacArthur for use in the Philippines and thirdly, that the invasion of Leyte be conducted at the earliest opportunity.

Nimitz in turn, quickly sent a message to the Joint Chiefs of Staff who were meeting in Quebec at the time for the Octagon Conference with President Franklin D Roosevelt and Prime Minister Winston Churchill. The Joint Chiefs, after consultation with General MacArthur and Admiral Nimitz, decided on the 14 September (the day before D-Day) that the landings on Leyte should be brought forward by two months, thus accepting the third point in Halsey's recommendations. Therefore Halsey cancelled the second phase of Stalemate II on 17 September, with the exception of the landing on Ulithi, which would be carried out by the 323rd Regimental Combat Team (RCT), 81st Infantry Division. The XXIV Corps was therefore transferred to MacArthur's command and landed on Leyte on 20 October 1944, fulfilling MacArthur's promise to return to the Philippines. The 1st Marine Division (under Major General William H Rupertus) would assault Peleliu as planned, with the remaining two regimental combat teams (RCT) of the 81st Infantry Division (under Major General Paul J Mueller) assaulting Angaur with the 322nd RCT (under Colonel Benjamin Venable) landing on Beach Red to the north and then push inland to the south and west. The 321st RCT (under Colonel Robert F Dark) would land on Beach Blue to the east and push west, tying in with the 322nd RCT. Upon completion, the 81st would revert to III Amphibious Corps reserve, garrisoning both Peleliu and Angaur after they were declared secure.

The 81st Infantry Division was made up of the 321st, 322nd and 323rd Regimental Combat Teams (RCTs) and was to assault both Angaur (321st and 322nd RCTs) and Ulithi (323rd RCT), but only when released by the 1st Marine Division commander. The 81st had been reactivated at Camp Rucker, Alabama in June 1942 after having previously served during World War One. It was raised from a small regular Army cadre from the 3rd Infantry Division, and filled out with newly commissioned reserve officers and conscripted troops. It received extensive desert training, participated in corps level exercises stressing the attack of fortified defensive positions, amphibious training in California, and was finally transferred to Hawaii where it undertook additional amphibious training. It was transported to Guadalcanal (by then a major US base) where it received jungle training, acclimatisation and training in rugged terrain. Angaur and Peleliu would be its first combat action.

Army infantry regiments had a 108-man headquarters with a platoon of three 37mm M3A1 anti-tank guns and a intelligence / reconnaissance platoon, a 118-man canon company with six 75mm M1A1 pack howitzers, a 165-man anti-tank company with nine 37mm anti-tank guns with a mine platoon, and a 115-man service company. It had three 871-man infantry battalions, each with a 155-man headquarters, three 193-man rifle companies and a 160-man heavy weapons company (D, H, M) with eight .30cal M1917A1 heavy machine guns in two platoons, and six 81mm M1 mortars in another. Each rifle company consisted of three 39-man platoons, each having three 12-man squads with a squad leader (M1 rifle), automatic rifleman (M1918A2 BAR), assistant automatic rifleman (M1 rifle), grenadier (M1 rifle, M7 grenade launcher) and seven rifleman (M1 rifles). The company had five 2.36in M1A1 bazookas and a weapons platoon with a section of two .30cal M1919A4 light machine guns and a section of three 60mm M2 mortars. Army divisional artillery was organised in a different way to that of the Marines, in that it was commanded by a Brigadier General, had three 105mm M2A1 howitzer battalions (316th, 317th and 906th Field Artillery Battalions) and one 155mm M1A1 howitzer battalion (318th). Each battalion had a headquarters and headquarters battery, service battery and three howitzer batteries with four tubes apiece. The divisional tank battalion was the 710th had four companies, three with seventeen M4A1 Sherman tanks (three platoons of five and two in the headquarters), and a fourth with 3in gun armed M10 tank destroyers. It also had six 75mm M8 self-propelled howitzers in the assault gun platoon attached to the headquarters.

On 16 September (D+1) General Rupertus signalled Lt General Geiger that Peleliu would be taken "in a few more days". Geiger therefore ordered the assault on Angaur to proceed. F-Day (as opposed to D-Day for Peleliu) was set for 17 September. Angaur lies some seven miles southwest of Peleliu and was defended by Major Ushio Goto with the 1st Battalion, 59th Infantry Regiment (Reinforced) that had been detached from the 14th Division. The island had been divided into four defence sectors with a small central reserve. The Japanese had expected the Americans to land on the excellent beaches to the southeast of the island, codenamed Beaches Green 1 and 2 by the Americans, and so this is where Goto build his strongest defences. Unfortunately for him, all this effort was spotted by reconnaissance overflights some months earlier and so the plan was to land on the beaches to the north (Red) and east (Blue). Angaur itself is some 5,000 yards long and 4,000 yards wide and is generally flat with a series of ridges in the northwest corner. It was the site of a large-scale mining operation, being the primary source of phosphate in the area.

The 322nd RCT landed on Beach Red and the 321st RCT landed on Beach Blue, both after a preliminary bombardment by the battleship, USS Tennessee, as well as one heavy cruiser and three light cruisers. Dauntless dive bombers from the USS Wasp (CV-18) also attacked the beaches and LCI(G)s (Landing Craft, Infantry (Gun)) fired 4.5in rockets to support the landing. Both encountered only minor resistance on landing but 321st RCT met stiffer resistance once it moved off the beach. The 323rd RCT conducted a feint off Beach Green.

Soon after landing, the 'Wildcats' found themselves caught up in dense terrain, infested with Japanese machine gun nests and snipers. By the end of the day both RCTs had met their objectives but at a heavy cost. The Japanese conducted a number of well-planned counterattacks during the night, which caused the Americans to withdraw from some of their more exposed positions, but all were eventually beaten back. F+1 saw both RCTs advance toward each other with tank and air support with the right flank of 321st RCT making contact with the left of 322nd RCT. The 322nd then pushed west from Beach Red towards the centre of the island and the high ground where Major Goto had his final defensive position, and had reached the phosphate plant on the outskirts of Saipan Town (the island's capital) early in the afternoon. The 321st RCT pushed west from Beach Blue and progressed well until they came up against the formidable defences of Beach Green.
F+2 (19 September) saw the 321st RCT continue to push west and attack the Beach Green defences from the rear, reducing the fortifications one by one. After hard fighting, the 321st RCT pushed on southwest stopping just short of the shoreline. This meant that there were only two remaining pockets of Japanese resistance left on Angaur - the largest being centred on Romauldo Hill, a series of coral ridges in the northwest. With the situation apparently under control, the III Amphibious Corps reserve, the 323rd RCT was sent on its way to occupy Ulithi.

F+3 saw the 321st RCT reduce the final pocket of Japanese resistance in the south and the 322nd RCT begin its assault on Romauldo Hill. It would in fact take over four weeks and bitter hand-to-hand fighting, using flamethrowers, grenades and demolition charges to extinguish Japanese resistan ce. Major Goto was killed on 19 October while fighting for one of the many cave complexes and Japanese resistance ended some three days later. F+3 heralded Major General Geiger contacting Major General Mueller about the need to deploy an RCT to Peleliu to relieve the 1st Marines. Mueller replied that the 321st could be despatched almost immediately after it had been reorganised and therefore began moving to Peleliu the next day. The 20 September saw Angaur being declared secure (despite the 322nd RCT having to clear the Romauldo Hill pocket) at 10.34.

Overall casualties for the 81st were comparatively light compared to what the 1st Marine Division was to suffer on Peleliu - 260 killed, 1,354 wounded and 940 incapacitated due to non-combat reasons. The Japanese lost some 1,338 killed and 59 captured.

370 miles to the northeast, saw the landing of the 323rd RCT (something the 321st had originally been slated to do) on Ulithi Atoll on 22 September (J-Day). They found the island's seaplane base and airfield deserted and carried out searches on a number of other islands in the area including Ngulu Atoll (16 - 17 October), Pulo Anna Island (20 November), Kayangel Atoll (28 November - 1 December) and Faris Island (1 - 4 January 1945). Here they found a small party of seventeen Japanese, eight of whom were killed, three escaping by boat, four escaped into hiding and two taken prisoner. American losses were two killed and five wounded. Ulithi was turned into a major fleet anchorage due to its excellent natural harbour, along with a Marine airbase and Naval seaplane base.